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It was previously shown that a temperature‐sensitive mutant of Sendai virus, ts‐23, readily establishes persistent infection in Vero cells at 37 C, a permissive temperature for growth of the mutant.

Bovine parainfluenza virus type 3 (PIV‐3) has a buoyant density of 1.197. The RNA of PIV‐3, like that of Sendai virus, is a single continuous chain which lacks polyadenylic acid sequences and tends to self‐anneal to a marked extent. It has a sedimentation coefficient of 42S and a molecular weight of 4.5 × 106, being slightly smaller than Sendai virus RNA (47S, 5.3 × 106).

Sixteen temperature‐sensitive mutants of Sendai virus were isolated from mutagenized stocks (10 mutants, designated numerically) and persistently infected cultures (6 mutants, designated alphabetically).